a. Field of the Invention
This invention relates generally to distance gauges or range finder devices and more particularly to a microwave depth gauge adapted to be mounted in a manhole or the like.
b. Description of the Prior Art
One of the direct methods of determining the flow rate of sewage in open sewage lines of known capacity is to determine the height or level of sewage in an open pipe or flume of known dimensions. A direct method of measuring sewage level is to remove a manhole cover and lower a "bob" or float down to the sewage by means of a calibrated rope. When the bob reaches the surface of the sewage the rope slackens, indicating to the person taking the sewage level reading that the amount of rope already played out represents the distance between street level and the upper surface of the sewage. Since the ultimate depth of the sewage line is known, the person taking the measurement can calculate the depth and volume of the sewage.
A known device for measuring sewage levels includes an automated mechanism including a moisture sensor attached to the end of a reel of wire. Periodically, the mechanism unreels the wire until the moisture sensor signals that the upper surface of the sewage has been reached. At that moment, the mechanism records the number of feet of wire played out and reels the sensor back in.
Microwave gauges for use in measuring fluid levels in tanks are known. An example of such a device is found in the disclosure of K. Edvardsson in U.S. Pat. No. 4,044,355 which teaches the use of microwave radiation to measure the fluid contents of a tank or similar enclosure. Edvardsson's device radiates a microwave signal that varies linearly in frequency through a sweep period and which is reflected back to the device from the upper surface of the fluid. A mixer is used to mix the radiated microwave signal with the reflected microwave signal to produce a difference signal having a frequency that is proportional to the distance to the upper surface of the fluid. Compensating circuitry is included in Edvardsson's device to insure an averaging of the difference signal to eliminate transients caused by floating objects, wave action, wall reflections, etc.
A problem with sewage and fluid level measurement apparatus found in the prior art is that the cover of a sewage line manhole needs to be either removed or elevated so that the measurement apparatus may be positioned for use. Obviously, should the removal of a manole cover take place in a crowded thoroughfare, such as a street or sidewalk, traffic could be obstructed.
In U.S. Pat. No. 3,798,848 of B. Campagna a "support ring" is disclosed which attaches to the inner rim of a manhole to support venting valves below a manhole cover. Because sewage and fluid level measurement apparatus is usually bulky, special support structures are needed which allow replacement of the manhole cover while the measurement apparatus is supported in the upper rim of the manhole.
It is an object of this invention to provide an apparatus for measuring sewage levels that can be supported by the rim of a manhole in such a manner that the manhole cover can be removed and replaced without interference.
It is another object of this invention to provide such an apparatus which further utilizes reflected microwave energy to determine the sewage level.